The present invention relates to a method of producing a lead-containing complex oxide without using any alkali metal hydroxide as a mineralizer.
The lead-containing complex oxide, particularly lead zirconate titanate Pb(Zr,Ti)O.sub.3 (PZT), is a well known ferroelectric ceramic material with good piezoelectric properties, and have been applied to various fields such as dielectric ceramics, piezoelectric ceramics, pyroelectric ceramics, resistive ceramics, semiconductive ceramics, etc. mainly as a piezoelectric element for SAW filter, sensor, actuator, etc., capacitor, nonvolatile memory (FERAM), etc.
PZT has been synthesized by various methods such as solid phase reaction, liquid phase reaction, sol-gel process and hydrothermal synthesis.
Since the solid state reaction is conducted at higher temperatures, Pb showing a low vapor pressure is vaporized during the heating to change the composition of the reaction mixture, thereby failing to produce a lead-containing complex oxide of the desired composition. In addition, the complex oxide particles synthesized by this method are not uniform in shapes and the particle size distribution is deflected. In the liquid phase reaction, the components of the starting materials are coprecipitated with each other. Since the optimum pH for precipitation varies among the components, the pH control for the coprecipitation is quite difficult. An additional disadvantage of this method is the generation of hydrates, which requires an additional heat treatment at a high temperature. Since metal alkoxides are used as the starting materials, the sol-gel process requires the use of organic solvents. In addition, since a heating process at a high temperature is needed, the compositional change of the reaction mixture occurs due to the vaporization of Pb thereby failing to produce a lead-containing complex oxide of the desired composition.
Japanese Patent Laid-Open No. 63-85014 proposes a method of producing PZT powders where a water-soluble lead salt, a water soluble titanium salt and a water soluble zirconium salt are dispersed in an aqueous alkaline solution containing an alkali metal hydroxide, and then subjected to hydrothermal reaction at 50-300.degree. C. The PZT powder produced by this method contains about 0.01-0.07% alkali metal such as K and Na derived from the mineralizer used even after the powder is thoroughly washed with water, etc. Most of the alkali metal adheres to the surface of the PZT particles. However, since a part thereof enters into the crystalline structure of PZT, the amount of the residual alkali metal in the PZT powder is very difficult to be further reduced. The addition of a metal ion, such as an alkali metal ion, having a valence different from that of the constituent metal ions of PZT causes the change of valence of the constituent metal ion and the existence of oxygen vacancies in the crystalline structure of PZT. As a result thereof, a PZT ceramic becomes semiconductive and has a reduced insulating property. In addition, the residual alkali metal serves as an acceptor to increase the coercive electric field and lower the insulating property of PZT to result in the reduction or loss of electric properties such as piezoelectric effect, etc. Thus, the use of alkali metal hydroxide needs an additional washing step for removing the alkali metal from the PZT powder, and involves a problem of reduction in electric properties due to the residual alkali metal because the alkali metal is not sufficiently removed by the additional washing step.
Japanese Patent Laid-Open No. 2-212316 discloses a method of producing PZT powders where the PZT is produced through a step of precipitating a lead-containing hydroxide from a reaction between a water-soluble lead compound and an aqueous solution of alkali metal with a low concentration, and a subsequent step of hydrothermal reaction. However, this method fails to prevent the alkali metal from remaining in the PZT powder.